1. Field of the Invention
The present description is related to the field of integrated circuits, and more specifically to devices, systems, and methods for generating a reference current from a voltage differential having a low temperature coefficient.
2. Description of the Related Art
A number of integrated circuits require a current reference for biasing various operations. For example, Radio Frequency IDentification (RFID) systems may be integrated circuits, and typically include RFID tags and RFID readers. RFID readers are also known as RFID reader/writers or RFID interrogators. RFID systems can be used in many ways for locating and identifying objects to which the tags are attached. In earlier RFID tags, the power management section included an energy storage device, such as a battery. RFID tags with an energy storage device are known as active tags. Advances in semiconductor technology have miniaturized the electronics so much that an RFID tag can be powered solely by the RF signal it receives. Such RFID tags do not include an energy storage device, and are called passive tags.
RFID systems are particularly useful in product-related and service-related industries for tracking large numbers of objects being processed, inventoried, or handled. In such cases, an RFID tag is usually attached to an individual item, or to its package.
In principle, RFID techniques entail using an RFID reader to interrogate one or more RFID tags. The reader transmitting a Radio Frequency (RF) wave performs the interrogation. A tag that senses the interrogating RF wave responds by transmitting back another RF wave. The tag generates the transmitted back RF wave either originally, or by reflecting back a portion of the interrogating RF wave in a process known as backscatter. Backscatter may take place in a number of ways.
The reflected-back RF wave may further encode data stored internally in the tag, such as a number. The response is demodulated and decoded by the reader, which thereby identifies, counts, or otherwise interacts with the associated item. The decoded data can denote a serial number, a price, a date, a destination, other attribute(s), any combination of attributes, and so on.
RFID tags may include a number of circuits, analog or digital, that are biased by a current reference. Reference current generators in integrated circuits, such as the RFID system, may be designed a number of ways known in the art. Prior art reference current generators typically generate currents that are proportional to absolute temperature (“PTAT”), and therefore currents that increase as temperature increases.
FIG. 1 is a diagram of a prior art reference current generator circuit 123 that includes a bias circuit 110 for providing drain currents to a pair of NMOS transistors 114 in a current mirror configuration. Reference currents are generated by the reference current generator circuit 123 by coupling a P-type mirror circuit 121P and an N-type mirror circuit 121N to each of the respective drains of the transistors 114 at nodes 113 and 117.
More specifically, the bias circuit 110 includes a pair of PMOS transistors 112, sourced by a voltage supply VDD, whose gates are coupled to each other and to the drain of one of the transistors 112 at node 113. Each of the drains of the transistors 112 are coupled to the drains of the transistors 114, whose gates are coupled to each other and to the drain of one of the transistors 114 at the node 117. Therefore, the drain current through the node 117 determines the gate-to-source voltage for both devices. The sources of the transistors 114 are coupled to ground, one of which is coupled to ground through a resistor 116. The transistor 114 coupled to node 113 is designed to have a smaller gate-to-source voltage than the transistor 114 coupled to node 117. The voltage differential between the gate-to-source voltages of the transistors 114 is thus the voltage across the resistor 116. The transistors 114 are similar devices and typically designed to have the same threshold voltage VT1. Because the transistors 114 have the same threshold voltage, the devices differ in size or current density to create the voltage differential necessary to provide the voltage drop across the resistor 116. The resulting resistor current through the resistor 116 is mirrored by the bias circuit 110 to determine the drain currents through the nodes 113, 117.
As current passes through the transistors 114, voltages VPGATE and VNGATE at nodes 113, 117 may respectively be used to drive one or more mirror circuits 121P, 121N for generating the reference currents. For example, a PMOS transistor 132 in the mirror circuit 121P may be biased by the VPGATE voltage at node 113 to generate a reference current IPREF sourced from VDD. Similarly, an NMOS transistor 134 in the mirror circuit 121N may be biased by the VNGATE voltage to generate another reference current INREF. The IPREF and INREF currents may be used to bias other circuitry, for example, components in the RFID system.
A problem with the prior art reference current generator circuit 123, however, is that the generated reference current increases as temperature increases due to the currents being directly proportional to temperature. As a result, the current references generated by the prior art reference current generator 123 may vary by more than 45% between a wide range of temperatures −40° C. to +65° C.
FIG. 2 is an illustration of the signal responses of the transistors 114 to temperature changes (ranging between −40° C. to 90° C.) in the prior art reference current generator circuit 123 of FIG. 1. An upper signal diagram 240 shows the gate-to-source voltage of the transistor 114 coupled to node 117 as a function of the drain current. A middle signal diagram 250 shows the gate-to-source voltage of the transistor 114 coupled to node 113 as a function of the drain current. In both cases, an increase in temperature causes the gate-to-source voltages of devices such as the transistors 114 to decrease, as is well known in the art. In the upper signal diagram 240, the gate-to-source voltage at a lower temperature −40° C., shown as a signal 245, decreases as the temperature increases to 25° C., shown as a signal 247. The more the temperature increases, the voltage continues to decrease, as shown by the current signal 249 at the higher temperature 90° C. At any given drain current in the middle signal diagram 250, the gate-to-source voltage is less than the gate-to-source voltage at the corresponding drain current in the upper signal diagram 240 so that the difference between the gate-to-source voltage creates the necessary voltage drop across the resistor 116.
Lowering the current density, however, causes a greater gap, as temperature increases, in the spacing between the gate-to-source voltage signals 255-259 of the middle signal diagram 250, as compared to the signals 245, 247, 249 of the upper signal diagram 240. Consequently, the change in voltage difference between the signal 255 at the temperature −40° C. and the signal 257 at the temperature 25° C. is greater than the corresponding voltage/temperature signals 245, 247 of the upper signal diagram 240. Because essentially identical transistors 114 are used, and because the transistors 114 are biased at different current densities to have different gate-to-source voltages, thus creating the voltage drop across the resistor 116, the transistors 114 have different temperature coefficients. Therefore, the same difference between transistors 114 to create the voltage differential creates a difference in the temperature variation between the signals of the upper signal diagram 240 and the middle signal diagram 250. Thus the voltage across the resistor 116 is shown in a lower signal diagram 260 to have PTAT characteristics, where the voltage represented by signals 265, 267, 269 increase as the temperature increases from −40° C. to 25° C. and 90° C., respectively.
A consequence of creating the voltage drop across the resistor 116 in the prior art reference current generator circuit 123 is the undesirable increase in the resistor voltage as temperature increases. As a result, the prior art reference current generator circuit 123 provides reference currents that are temperature dependent. The high current variation of the reference currents (by 45%) increases power consumption and degrades performance. For example, sensitivity is a critical parameter particularly in RFID systems, since passive tags rely on power from readers antennas to operate. Any undesirable variation in the reference current due to temperature, and thus an increase in power consumption, limits the reliability and performance of RFID tags.
There is therefore a need for a reference current generator circuit having currents with substantially the same temperature coefficient such that the temperature variation of the voltage differential is reduced and a reference current may be generated that maintains a substantially constant current over a wide range of temperatures.